A.1 Case Study: Mixed Parallel/Serial Homogeneous Cluster
A multi-user site wishes to control the distribution of compute cycles while minimizing job turnaround time and maximizing overall system utilization.
The groups 'Meteorology' and 'Statistics' must receive approximately 45% and 35% of the total computing resources respectively. Nodes cannot be shared among tasks from different jobs.
The system should attempt to minimize turnaround time during prime hours (Mon - Fri, 8:00 AM to 5:00 PM) and maximize system utilization during all other times. System maintenance should be scheduled efficiently within these constraints.
The network topology is flat and nodes are homogeneous. This makes life significantly simpler. The focus for this site is controlling distribution of compute cycles without negatively impacting overall system turnaround and utilization. Currently, the best mechanism for doing this is Fairshare.
Fairshare can be used to adjust the priority of jobs to favor or disfavor jobs based on fairshare targets and historical usage. In essence, this feature improves the turnaround time of the jobs not meeting their fairshare target at the expense of those that are. Depending on the criticality of the resource distribution constraints, an allocation bank such as Gold, which enables more stringent control over the amount of resources which can be delivered to various users, may be desirable.
To manage the prime time job turnaround constraints, a standing reservation would probably be the best approach. A standing reservation can be used to set aside a subset of the nodes for quick turnaround jobs. This reservation can be configured with a time based access point to allow only jobs which will complete within some time X to utilize these resources. The reservation has advantages over a typical queue based solution in this case in that these quick turnaround jobs can be run anywhere resources are available, either inside, or outside the reservation, or even crossing reservation boundaries. The site does not have any hard constraints about what is acceptable turnaround time so the best approach would probably be to analyze the site's workload under a number of configurations using the simulator and observe the corresponding scheduling behavior.
For general optimization, there are a number of scheduling aspects to consider, scheduling algorithm, reservation policies, node allocation policies, and job prioritization. It is almost always a good idea to utilize the scheduler's backfill capability since this has a tendency to increase average system utilization and decrease average turnaround time in a surprisingly fair manner. It does tend to favor somewhat small and short jobs over others which is exactly what this site desires. Reservation policies are often best left alone unless rare starvation issues arise or quality of service policies are desired. Node allocation policies are effectively meaningless since the system is homogeneous. The final scheduling aspect, job prioritization, can play a significant role in meeting site goals.
To maximize overall system utilization, maintaining a significant Resource priority factor will favor large resource (processor) jobs, pushing them to the front of the queue. Large jobs, though often only a small portion of a site's job count, regularly account for the majority of a site's delivered compute cycles. To minimize job turnaround, the XFactor priority factor will favor short running jobs. Finally, in order for fairshare to be effective, a significant Fairshare priority factor must be included.
For this scenario, a resource manager configuration consisting of a single, global queue/class with no constraints would allow Moab the maximum flexibility and opportunities for optimization.
The following Moab configuration would be a good initial stab:
# reserve 16 processors during prime time for jobs requiring less than 2 hours to complete SRNAME fast SRTASKCOUNT 16 SRDAYS MON TUE WED THU FRI SRSTARTTIME 8:00:00 SRENDTIME 17:00:00 SRMAXTIME 2:00:00 # prioritize jobs for Fairshare, XFactor, and Resources RESOURCEWEIGHT 20 XFACTORWEIGHT 100 FAIRSHAREWEIGHT 100 # disable SMP node sharing NODEACCESSPOLICY DEDICATED
Group:Meteorology FSTARGET=45 Group:Statistics FSTARGET=35
MonitoringThe mdiag -f command will allow you to monitor the effectiveness of the fairshare component of your job prioritization. Adjusting the Fairshare priority factor will make fairshare more or less effective. A trade off must occur between fairshare and other goals managed via job prioritization.
The mdiag -p command will help you analyze the priority distributions of the currently idle jobs. The showstats -f AVGXFACTOR command will provide a good indication of average job turnaround while the showstats command will give an excellent analysis of longer term historical performance statistics.
ConclusionsAny priority configuration will need to be tuned over time because the effect of priority weights is highly dependent upon the site specific workload. Additionally, the priority weights themselves are part of a feedback loop which adjust the site workload. However, most sites quickly stabilize and significant priority tuning is unnecessary after a few days.
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